1. Technical Field
This invention relates to the technology of mounting wheels with a braking assembly, and in particular, to techniques for eliminating dis-alignment that may develop between the wheel hub and the braking element of the braking assembly, sometimes called lateral run-out, which may cause pulsation of the brake pedal when applied or may cause undue brake wear.
2. Discussion of the Prior Art
It has become common practice to delete tapered roller bearing packs at the connection between hubs and braking elements (i.e. a cast hollow rotor plate) and substitute a metal surface to surface flange mount of the braking element to the hub inner-hat flange. The braking element or rotor is designed to operate in a precise plane normal to the axis of the wheel; there is little tolerance for deviation of such plane which deviation must generally be no greater than 0.001 inch at the rotor outer diameter to avoid lateral run-out. To assure such alignment, the mating flange surfaces of the mount are machined to very high accuracy and, when brought together under the coupling force of threaded fasteners, will achieve such alignment. The braking caliper that operates on such rotor, depends on the rotor remaining in the designed rotational plane relative to the wheel axis for the caliper brake pads to operate with a predetermined movement. If not, the brake pedal feel will be different and the driver will sense pulsating contact by the brake pads as the skewed rotor wobbles between the pair of brake pads.
Such dis-alignment can readily arise from the presence of corrosion between the mating mounting surfaces. Corrosion will appear as discontinuous non-uniform particles that may force the flanges apart as little as point 0.0005 inches to create some degree of lateral rotor run-out. This is as little as a 0.01% deviation of the rotor outer diameter from its intended rotational plane. Such run-out creates an apparent cycle of contact and non-contact between the rotor and a brake pad while the wheel is rotating even without the brakes being applied. The driver interprets this as braking system roughness (pedal pulsation) or a sense of steering wheel shake, both feelings occurring when braking is applied at vehicle speeds of around 30-70 mph.
The obvious attempt to solve this problem is to apply a coating to the metal (i.e., steel) mounting surfaces that would inhibit corrosion. Unfortunately, introducing typically obvious materials to the assembly results in either (i) the material being too easily removed or migrated during normal micro shifting of the hub and rotor flanges when load and under centrifugal stress, or (ii) the material becomes too hard and frets under use resulting again in uneven distribution of the coating which becomes equivalent to a disturbance like corrosion. In either case there is progressive evolution of discontinuity between the mating surfaces on a microscopic scale resulting in lateral rotor run-out. Fretting of the coatings, that eventually become hard, is due to microscopic movement of the two mating surfaces during use that grinds the coating or the material may become unstable at temperatures such as 300-600.degree. F. to locally become patchy.